1. Field of the Invention
The present invention relates to a controlling method and apparatus for actuating an induction motor, and more particularly to an actuating control method for an induction motor suitably applied for controlling a main shaft of a machine tool, which requires a predetermined output in a wide speed region ranging from a low speed to a high speed.
2. Description of the Related Art
A recent induction motor is designed so that a large torque can be obtained in a low speed region with the same primary current value by use of a switching control of primary windings and a slip control.
That is, in a conventional switching operation for the primary windings, there are provided two terminals so that primary windings of Y--Y connection of respective phases are supplied with exciting current from these two terminals. A first terminal is provided to fully supply exciting current to primary winding of each phase for rotation. While a second terminal is provided at an intermediate point of each primary winding so as to supply the exciting current to a part of each primary winding.
Then, the switching operation of the primary windings is carried out in such a manner that electric power is only supplied from the first terminal in the case where a rotational speed of the motor is low, so that each phase winding is fully supplied with exciting current. And, electric power is only supplied from the second terminal in the case where the rotational speed of the motor is high, so that each phase winding is partly supplied with exciting current.
In the case where the motor is controlled through the switching operation of the primary windings as is described above, there was a problem such that an undesirable drop of output occurs when the primary winding is switched from a low-speed winding to a high-speed winding or vice versa where the extent of a rotational speed region exceeds a certain limit, thereby causing a problem such that a constant output cannot be obtained throughout a wide speed region.
FIG. 4 (PRIOR ART) is a graph showing an output characteristics of the motor in relation to its rotational speed, where the actuation of the motor is controlled on the basis of such a switching operation of the primary windings. As shown in FIG. 4, an output of the motor will not increase proportionally even when the rotational speed of the motor is increased to a relatively high speed range as far as the motor is actuated by the low-speed windings.
Thus, even when the winding is switched to the high-speed windings from the low-speed windings, an output necessary for obtaining a predetermined torque will not be generated as far as the rotational speed of the motor is within the relatively low-speed range of the speed region available by the high-speed windings. For this reason, there will occur an output reduction whose phenomenon is shown in FIG. 4 as area a filled with slant lines.